The MiT transcription factor family contains four bHLHzip proteins which are critical for development of several cell lineages and exhibit overlapping DNA recognition with the Myc family. One member, Miff, is regulated by Melanocyte Stimulating Hormone (MSH) to stimulate pigment cell growth and differentiation. Another, TFE3, modulates osteoclast development together with Mitf. We recently cloned a recurring translocation in Papillary Renal Cell Carcinoma (PRCC), and identified the MiT family member TFEB as a new fusion oncogene, which places full length TFEB under the transcriptional regulation of a ubiquitous and abundant gene of unknown function reminiscent of c-Myc in Burkitt's lymphoma. Translocations which fuse another MiT factor, TFE3, also occur in PRCC as well as Alveolar Soft Part Sarcomas. The oncogenicity of transcriptionally dysregulated MiT factors sparked investigation of Clear Cell Sarcoma (CCS), an EWS-ATF1 translocated tumor which inexplicably expresses melanoma markers. EWS-ATF1 was seen to constitutively upregulate Miff via mimicking Melanocyte Stimulating Hormone signaling in melanocytes. Dysregulated Mitf, in turn, drives CCS growth and survival. All known MiT-dysregulated tumors are uniformly resistant to conventional chemotherapy. This proposal examines this growing family of malignancies to dissect the pathways through which they transform and identify diagnostic markers and therapeutic targets using this information. In Specific Aim 1 we employ disruption-rescue assays we developed to determine structural and post-translation requirements of MiT factors for specific oncogenic behaviors. In Specific Aim 2 we ask whether the overlapping DNA recognition properties of the MiT and Myc families reflect overlapping transformation mechanisms. Specific Aim 3 uses candidate- and microarray approaches to identify transcriptional targets of the MiT family in these cancers, and scrutinizes their functional importance in tumorigenicity. One such newly identified MiT target gene is c-MET, a finding of particular interest because human germline c-MET mutations produce hereditary PRCC-- the same malignancy in which TFEB or TFE3 translocations occur for sporadic tumors, c-MET was seen to be superactivated in all 6 MiT-associated tumor lines examined, and will be studied in primary tumors. Small molecules inhibitors of c-MET exist and will be examined (together with molecular controls) as potential targeted therapy for these incurable cancers.